1. Field of Invention
Embodiments of the present invention generally relate to silica crucibles and methods of making the same. More particularly, embodiments of the present invention relate to silica crucibles with highly pure and bubble free inner crucible layers and methods of making the same.
2. Discussion of the Related Art
There are several methods for making single crystal silicon materials. One such process—the “Czochralski” (CZ) process—has been widely adopted for producing single crystal silicon materials for semiconductor applications including solar cell applications. In the CZ process, molten silicon is contained within a vessel and a tip of a single-crystalline seed crystal is dipped into the molten silicon. The seed crystal is then “pulled” while being rotated. As a result, a single crystal silicon ingot is grown from the molten silicon.
A crucible is one vessel commonly employed in this pulling operation for making silicon ingots. Crucibles are typically configured in a bowl shape to contain the molten silicon during the pulling operation. Crucibles are made of silica glass, or quartz glass, to introduce a desired amount of oxygen into the molten silicon while keeping if from contamination with metallic impurities.
During the CZ process, the inner surface of a silica glass crucible typically dissolves into the molten silicon. Any bubbles present in the inner crucible layer may open to the melt by dissolution and are a potential source of particles that may disturb the single-crystalline structure of the silicon ingot. Therefore, crucibles are typically manufactured to have a bubble-free inner crucible layer. The outer crucible layers are typically manufactured to contain numerous tiny bubbles to facilitate scattering of radiation from heater elements.
To minimize the possibility of contamination of the silicon ingot during the CZ process, the inner crucible layer should be as pure as possible.
Silica glass made from natural quartz grain often contains an Al content of 5-20 ppm. The Al element in the silica glass is often associated with positively charged metallic impurities such as Li, Ca and Na. Therefore, reduction of Al content is desirable to minimize concentration of other metallic elements.
One known method of manufacturing silica glass crucibles or quartz glass crucibles, is to form silica grain, which can be synthetic silica grain or natural quartz grain, in a crucible shape inside of a rotating mold, then fuse it into a silica glass crucible. The crucible is taken out of the mold and mechanically finished and cleaned to a final product.
During fusion, silica sublimates from the inner surface of the inner crucible layer. As silica sublimates faster than alumina, when natural quartz grain is used as the starting material, aluminum is left on the inner surface of the inner crucible layer. As mentioned above, aluminum is associated with other metallic impurities.
One method disclosed in Japanese Patent JP 63-166791, is understood to minimize the Al content at the inner surface of the inner crucible layer by etching the surface of the inner crucible layer with hydrofluoric acid. This method, however, is not economical or environmentally preferable due to the use of hydrofluoric acid.
Another known method of manufacturing quartz glass crucibles, disclosed in U.S. Pat. No. 6,510,707, is understood to make the inner crucible layer bubble-free with little bubble growth and reduce the Al content at the inner surface of the inner crucible layer. A crucible is fused by employing a relatively large amount of electrical power to reduce bubble and bubble growth leaving an aluminum-rich layer on the surface of the inner crucible layer. The aluminum-rich layer is then removed by abrasive processes or etching processes. While this method can produce an inner crucible layer that may be relatively bubble-free with little bubble growth during the CZ process, abrading or etching the aluminum rich layer is time consuming and not economical.
Consequently it would be desirable to prepare a silica crucible having an inner crucible layer with minimal impurity concentration and reduced bubble and bubble-growth at reasonable cost.